A heavy period usually comes down to a hormonal imbalance, a structural change in the uterus, or an underlying health condition affecting how your blood clots. The clinical threshold is soaking through one or more pads or tampons every hour for several hours in a row, but you don’t need to hit that mark to have a problem worth investigating. If your periods have changed noticeably, or if heavy bleeding is disrupting your daily life, there’s almost always an identifiable reason.
How Hormones Cause Heavy Bleeding
The most common cause of unusually heavy periods is a hormonal imbalance between estrogen and progesterone. During a normal cycle, estrogen thickens the uterine lining in the first half, then progesterone stabilizes it after ovulation. If you don’t ovulate in a given cycle, progesterone never kicks in. Without that counterbalance, estrogen keeps building the lining unchecked, and when it finally sheds, there’s simply more tissue and blood to come out.
This pattern, sometimes called estrogen dominance, is especially common at two stages of life: the first few years of menstruating, when cycles haven’t become regular yet, and during perimenopause. In perimenopause, declining estrogen throws off the balance with progesterone, and ovulation becomes less predictable. Some cycles you may ovulate normally, others you won’t, which is why periods can swing between light and flooding from month to month. Conditions like polycystic ovary syndrome (PCOS) can also cause chronic anovulation and heavy bleeding at any age.
When the lining grows too thick without being shed regularly, the condition is called endometrial hyperplasia. The cells that make up the lining crowd together and can become abnormal over time, which is one reason persistent heavy bleeding is worth getting checked out rather than just endured.
Structural Changes in the Uterus
Physical changes inside or around the uterus are another major category. Fibroids, which are noncancerous growths in the uterine wall, are extremely common and can range from pea-sized to large enough to distort the shape of the uterus. Fibroids that grow into the uterine cavity or sit within the muscular wall tend to cause the heaviest bleeding because they increase the surface area of the lining and interfere with the uterus’s ability to contract and clamp down on bleeding vessels.
Polyps are smaller, finger-like growths on the uterine lining that can also cause heavy or prolonged bleeding. They’re softer than fibroids and more likely to cause irregular spotting between periods in addition to heavier flow.
Adenomyosis is a condition where tissue that normally lines the uterus grows into the muscular wall itself. This makes the uterus enlarged and boggy, and it struggles to contract effectively during your period. The result is heavy, prolonged bleeding often accompanied by severe cramping. Adenomyosis is more common in women in their 30s and 40s and is frequently underdiagnosed because its symptoms overlap with fibroids.
Bleeding Disorders and Clotting Problems
Some people bleed heavily because their blood doesn’t clot the way it should. Von Willebrand disease, an inherited condition that affects a protein needed for clotting, is found in 5% to 24% of women with chronic heavy menstrual bleeding. Many of these women go years without a diagnosis because heavy periods are often dismissed as “just how their body works.” If you’ve had heavy periods since your very first cycle, bruise easily, or have prolonged bleeding after dental work or minor cuts, a clotting disorder is worth considering.
Prevalence varies by ethnicity. Research from ACOG shows von Willebrand disease is more common among white women with heavy bleeding (about 16%) compared to Black women (about 1.3%). Other platelet disorders and rare clotting factor deficiencies can also be responsible.
Thyroid Problems and Other Medical Conditions
Your thyroid gland, which regulates metabolism, also influences how well your blood clots. When thyroid hormone levels are low (hypothyroidism), the body shifts toward a state where clotting factors are reduced. Specifically, low thyroid hormones lead to lower levels of key clotting proteins and a higher incidence of acquired von Willebrand syndrome. The result is heavier and longer periods. Thyroid issues are easily detected with a blood test and treatable, making this one of the more straightforward causes to rule out.
The Copper IUD Effect
If your periods became noticeably heavier after getting a copper IUD, you’re not imagining it. Studies measuring actual blood loss found that menstrual volume increased by 54% to 59% after insertion of a copper IUD. Average blood loss went from about 59 ml before insertion to around 91 ml at three months, and it stayed at that level through the first year. This increase is a well-documented side effect of copper IUDs specifically (hormonal IUDs typically make periods lighter). For most people the heavier flow is manageable, but for some it’s enough to cause iron deficiency over time.
When Heavy Periods Lead to Anemia
The biggest practical consequence of ongoing heavy periods is iron deficiency anemia. Every period removes iron from your body, and if you’re losing more blood than average, you can deplete your iron stores faster than diet alone can replace them. The signs are often subtle at first: fatigue that doesn’t improve with sleep, feeling short of breath going up stairs, brain fog, cold hands and feet, and unusually pale skin or nail beds.
Iron deficiency is defined as a ferritin level below 30 ng/mL. Anemia sets in when hemoglobin drops below 12 g/dL in women. Both are detected with a simple blood draw. If you’ve been living with heavy periods for years, it’s worth getting these checked even if you feel “fine,” because your body adapts gradually to lower iron levels and you may not realize how much better you could feel.
How Heavy Bleeding Gets Diagnosed
Figuring out the cause of heavy periods typically involves a combination of blood work and imaging. Blood tests check for iron deficiency, thyroid disorders, and clotting problems. An ultrasound is usually the first imaging step, using sound waves to look for fibroids, polyps, or signs of adenomyosis. If more detail is needed, a sonohysterography (where fluid is injected into the uterus to get a clearer ultrasound image) or hysteroscopy (where a thin camera is inserted through the cervix to look directly at the uterine lining) can identify growths or abnormalities that a standard ultrasound might miss.
An endometrial biopsy, where a small tissue sample is taken from the lining, may be recommended particularly for women over 35 or those with risk factors for endometrial hyperplasia. This checks for precancerous or abnormal cell changes. A Pap test may also be done to rule out cervical issues. None of these tests require general anesthesia, though some can cause brief cramping.
Perimenopause and Changing Patterns
If you’re in your late 30s or 40s and your periods have recently become heavier, erratic, or both, perimenopause is a likely explanation. This transition phase can last anywhere from a few years to a decade before menopause. As your ovaries produce less consistent hormone levels, some cycles may skip ovulation entirely, leading to a thicker lining and a heavier period when it finally arrives. You might have a light period one month and a flooding one the next.
That said, perimenopause is a diagnosis of exclusion. Fibroids, polyps, and thyroid issues all become more common in this age range too, so it’s important not to assume every change is “just perimenopause” without at least a basic workup. New-onset heavy bleeding after age 40 in particular warrants an evaluation to rule out endometrial hyperplasia or other structural causes.